The invention relates to a process for preparing organic hydroperoxides.
Processes for the preparation of organic hydroperoxides by oxidizing the corresponding hydrocarbons are known from U.S. Pat. Nos. 4,329,514; 4,404,406; 4,408,081; 4,408;082; European patents applications Nos. 584,956 and 567,336 and other documents. Tertiary-butyl hydroperoxide (TBHP) may be prepared this way, as well as hydroperoxides of cyclohexane, cumene and ethylbenzene. TBHP is of particular interest, as it can be used in the synthesis of propylene oxide (PO) and, via the intermediate tertiary butyl alcohol, of methyl tertiary-butyl ether (MTBE). An example of this synthesis is found in European patent application No. 416,744 and prior art discussed therein. The synthesis of organic hydroperoxides is not an easy one. Aside from (explosive) runaway reactions, also the lack of selectivity towards the desired organic hydroperoxide is an issue of major concern. A 100 percent yield is impossible due to the variety of oxidation reactions competing with each other. Besides, the yield is also affected by decomposition of the hydroperoxide. For instance, cumene hydroperoxide will decompose (rearrange) into phenol and acetone (cf. "Organic Chemistry" by Morrison and Boyd, 3rd ed., sec. 28.6). Similar reactions are known from "Advanced Organic Chemistry" by Jerry March, 3rd ed. (cf. reaction 8-23). This decomposition may be catalyzed by trace amounts of metal ions derived from the (chromium/steel) inner reactor walls (e.g., Fe2+ and Fe3+).
The first step as described in the art to improve the selectivity and reduce decomposition concerns treatment of the reactor walls, typically with sodium pyrophosphates as disclosed in U.S. Pat. No. 3,816,540, or with sodium stannate. Often the inner reactor walls are already passivated by the manufacturer prior to delivery. This method is effective, as removing the sodium pyrophosphates lowers the selectivity, which may be restored upon renewed passivation.
Another approach concerns the use of reactors that are entirely inert, such as, gold plated reactors. Increased selectivity's under comparable circumstances may be found. However, the prevailing reaction conditions will damage the gold plating, resulting in the loss of selectivity. As the manufacture and rejuvenation of gold plated reactors is highly expensive, this approach is not attractive.
Ideally the selectivity towards organic hydroperoxides is improved up to the rate achievable in a gold plated reactor, without the disadvantages mentioned above. This the inventors now have achieved with a relatively simple adaptation of known processes.